The ability to identify and/or track items such as packages and items of inventory is of great importance in industries such as manufacturing, distributing, and retail sales. Such industries are increasingly moving toward wireless systems that can automatically identify and track these items, as well as being able to communicate information regarding these items, such as identification information, destination, lot number, or even measured parameters such as temperature, lifetime, or status. One method for identifying items and providing information involves attaching a wireless communication device to an item or lot of items. This communication device can include a transponder, or transceiver, which can be easily attached to the item, such as in or on a label or tag for the item. The wireless communication device can respond to commands or instructions included in standard radio frequency (RF) signals.
One wireless communication device that is gaining in popularity is the Radio Frequency Identification (RFID) tag, which can include a transponder or transceiver as known in the art. RFID tags typically are placed onto items to be tracked, such as clothing items in a retail store. The store operates at least one RF zone used to communicate with the various RFID tags. An RF base station or RF reader typically is at the center of each RF zone, designed to generate a continuous wave electromagnetic field at a determined carrier frequency, which currently is 915 MHz in the United States for standard wireless communication. The electric field can be modulated in order to create a data signal that can be carried with the field, at a data rate that typically is lower than the carrier frequency. The modulated signal can be transmitted from an RFID base station. The signal then is received by an RFID tag in the RF zone, such that the tag can determine whether or not to respond, such as by reading an identification code or range of codes contained in the signal. If the RFID tag is to respond, the tag can generate a corresponding modulated RF signal, containing information stored within the tag or determined by the tag, and can transmit the modulated response RF signal to be received by the base station.
There typically are two types of RFID tags: active tags that include an RF transmitter and passive tags that do not include a transmitter, but rely upon modulated back-scattering to provide a return link to an interrogating base station. Passive tags can have an internal power source, such as a battery, or can obtain operating power by rectifying an RF signal transmitted from a base station. Each RFID tag typically has a minimum RF field strength requirement in order to read the incoming signal. An RFID tag powered by electromagnetic field can require substantially more power in the incoming signal than a system having an internal battery. Since field-powered RFID tags do not include a power source, however, these tags typically are much less expensive than active tags containing power sources. Further, field-powered tags are more reliable over time since there is no battery to lose power. RFID tags without power sources also can be simpler and cheaper to produce.
RFID systems have significant advantages over other existing identification and/or tracking systems, as the RFID systems have a rapid read rate, and can read tags that are at a distance from, and/or out of the line of sight of, the base station or tag reader. Further, some RFID tags allow for the updating of information stored within the tag. A typical RFID tag includes a chip coupled to an antenna, such as a dipole antenna as known in the art, formed on a substrate. A problem with such a device is that the orientation of the antenna with respect to the reader can greatly affect the readability of the RFID device, as well as the ability of the device to receive a signal. If the direction of the antenna is substantially parallel to the direction of the electromagnetic field, for example, the RFID device might not pick up a readable signal.
It would be advantageous to increase the reading distance and accuracy of the RFID tags as much as possible, in order to minimize the number of necessary readers and reduce the likelihood of data errors. It also would be desirable to limit the size of the RFID tags, in order to facilitate the application and storage of the tags as well as to lower manufacturing costs. Simply lengthening the antenna to increase the carrier frequency can result in a tag that is too large to be used with a label on a retail item. Further, the transmitting power cannot be increased simply to increase the read distance, as the maximum radiated power is limited by governmental regulation. It also would be advantageous to develop an RFID tag that can read and transmit in substantially any direction, allowing for orientation-independent operation of the RFID tag.